The invention relates to a process to recover liquid methane from a feed gas mixture comprising essentially methane, C.sub.2+ hydrocarbons, carbon dioxide and nitrogen.
Highly pure liquid methane is used to an increasing extent as a non-polluting fuel for diesel engines in locomotives, buses and trucks. The liquid methane is generally recovered from natural gas. For example, in U.S. Pat. No. 4,761,167, a process to recover methane from a gas mixture containing methane, C.sub.2+ hydrocarbons, carbon dioxide and nitrogen provides feeding the gas mixture first to a cryogenic distillation stage, in which the C.sub.2+ hydrocarbon fraction as well as a first carbon dioxide fraction are recovered. The remaining CH.sub.4 /N.sub.2 /CO.sub.2 gas mixture is then conveyed to a pressure swing adsorption stage where the mixture is freed from carbon dioxide. In a nitrogen separating unit downstream of the pressure swing adsorption stage, the resultant gas mixture is separated to provide the methane product fraction as well as a nitrogen fraction, and the latter is used to regenerate the adsorber loaded with carbon dioxide. This process is particularly useful if the nitrogen concentration in the feed gas mixture is approximately of the same order of magnitude as the methane concentration.
In an alternative to this process, there is employed, instead of the cryogenic distillation stage, an amine scrubbing stage to separate the carbon dioxide from the feed gas mixture. In this case, is necessary to employ a downstream adsorption unit for drying the resultant water-saturated feed gas mixture from the amine washing stage. This procedure has the drawback, however, that the adsorption unit, because of the complete water saturation of the gas stream exiting from the amine washing stage, must be very large. Furthermore, the system must provide for the disposal or recycling of the amine scrubbing agent used in the carbon dioxide separation stage.